1. Field of the Invention
The present invention relates to a plasma CVD method and a plasma CVD apparatus for film formation including a carbon protection film of a magnetic recording medium.
2. Description of the Prior Art
In a metal magnetic thin film type magnetic recording medium having a metal magnetic thin film as a magnetic layer, in order to improve the sliding property between the recording medium and a magnetic head, the metal magnetic thin film is normally covered with a hard type protection film.
Examples of this protection film include a carbon film, quartz (SiO.sub.2) film, zirconia (ZrO.sub.2) film, and the like. In particular, a great benefit can be expected from a diamond-like carbon (DLC) film having a diamond structure for its hardness and excellent protection property.
For forming this DLC film, for example, the plasma CVD (chemical vapor phase epitaxy) process is employed.
For forming a DLC film by way of the plasma CVD method, a tape-shaped plastic film (or support member) covered with a metal magnetic thin film is made to travel along a cylindrical can so as to pass over a reaction tube. This reaction tube is provided with a discharge electrode connected to a DC power source, so that a raw material gas introduced into the tube and passing this electrode is dissolved into plasma, generating plus ions. The plus ions generated here are directed toward a surface of the metal magnetic thin film traveling continuously along the cylindrical can and are accumulated there, forming a DLC film. It should be noted that the aforementioned cylindrical can has a width greater than the width of the support member so that the ends of the cylindrical can are exposed from both sides of the support member. This is to cope with any traveling fluctuation of the support member in the width direction as well as width fluctuations of the support member itself.
When forming a film by way of the plasma CVD method, the support member traveling over the reaction tube may be heated up to the order of 60 to 70.degree. C. by the plasma heat and the electrode heating. On the other hand, a plastic film made from, for example, polyethylene terephthalate or the like has a heat resistance of 60 to 70.degree. C., and if heated as has been described above, it may be deformed.
For this, in the plasma CVD method, such a thermal deformation is avoided by providing a cooling function in the cylindrical can and by forming the cylindrical can itself from a metal of a high thermal conductivity such as a stainless roll plated by chrome.
However, when a cylindrical can is made from a conductive body such as a metal, the exposed portions of the cylindrical can are easily subjected to the flow of plasma and plus ions, causing an abnormal discharge. As a result, there arise problems that the film formation is deteriorated and the cylindrical can is damaged.